The Machine Tool Monitor is a device which monitors vibration signals produced by the machining of metal parts and interprets patterns in these vibration signals to detect events of significance in the control of the machining process. Illustrative of such devices is that described in commonly assigned U.S. Pat. No. 4,724,524, Vibration-Sensing Tool Break and Touch Detector for Machining Conditions. This machine has two operating modes. In the tool break detection mode it detects cutting tool insert breaks; in the tool touch detection mode it detects the first contact of an advancing tool with the rotating workpiece.
After treatment of tool breakage and touch control, however, one of the greatest remaining problems in the development of such systems for controlling cutting tools for machining of rough stock workpieces is programming the allowance for wide dimensional variations. If the accepted variations are large, the cutting tool can start its operation on very rough stock, but a program which can accept rough stock having the maximum values will waste significant time "cutting air" on rough stock having the minimum values. If, however, the accepted variations are smaller in order to reduce the wasted time, tightening the tolerances on the rough stock significantly increases the cost of this stock.
Most modern machine controllers include routines for probing the outside dimensions of the rough stock. Existing probes are both expensive and delicate and are consequently frequently subject to damage when measuring the rough stock. Additionally, these probes only detect the point of initial contact with the surface of the stock to be machined and offer little or no information concerning the orientation, roughness, hardness, or variations in that surface.
Tool monitoring systems have employed an analysis of vibration signals produced by machining of metal parts to detect a tool break. However, none of these monitoring devices continuously control the operation of a cutting tool working on a workpiece. Thomas et al., in the above-mentioned U.S. Pat. No. 4,724,524, and also in U.S. 4,642,617, 4,806,914 and 4,636,780, for example, detect a waveform profile corresponding to a tool break. When the tool break is detected from a characteristic acoustic signature, this device simply stops the operation of the cutting tool. Begin, in U.S. Pat. No. 4,514,797 discloses a device that detects the increasing amplitude of the waveform produced between the cutting tool and the workpiece due to wear. However, Begin provides no teaching of any means to separate the vibrations of the cutting operations from the vibrations produced by the cutting machine itself. As has been pointed out by Thomas et al., without particularly sensing and monitoring the vibrations due only from the cutting operation, the control of fine machining cannot take place. Mention is also made of Moore, who, in U.S. Pat. No. 4,563,897, develops a machining "signature" waveform profile of the cutting tool in moving contact on the surface of the workpiece. A deviation from a preset value of the "signature" then corresponds to wear of the cutting tool. Moore's disclosure thus does not allow for wide variations in surface hardness and accepting wide tolerances of workpiece.
None of these patents teach or disclose actually controlling the cutting operation. Thus, no system exists for continuously controlling the machining of a workpiece which has wide dimensional tolerance.